The present invention relates to bearing materials for plain sliding bearings and particularly, though not exclusively, to so-called overlay alloy layers which are deposited on the surface of a second, thicker layer of bearing alloy.
Plain bearings such as half bearing shells for internal combustion engine crankshaft connecting rod and main bearings for example frequently comprise at least three distinct layers: a first layer of a strong supporting material such as steel; a second layer of a bearing alloy bonded to the backing layer comprising a copper-lead based alloy or an aluminium-tin based alloy for example; and, a third, thinner layer and generally softer than the second layer, of a so-called overlay alloy which endows the bearing with properties of inter alia conformability and dirt embedability.
Many known overlay alloy systems contain high proportions of lead and include such systems as lead-indium, lead-tin and lead-tin-copper for example. It is a possibility that environmental legislation will render the use of lead in overlay alloys unacceptable at some time in the future. The problem is two-fold: firstly; there is the problem of dealing with lead containing effluents and residues generated as a result of electro-deposition of the overlays from electrolytes; and secondly there is the problem of the lead containing bearings themselves which may present an environmental hazard when vehicles are scrapped.
Pure tin coatings have been used in the past as so-called sacrificial coatings on the surface of plain bearings. These coatings have been very thin, about 2 to 3 xcexcm, which is intended to wear away as a new engine xe2x80x9cbeds inxe2x80x9d. However, such sacrificial tin coatings have been deposited on top of other conventional overlay coatings such as those described above which are intended to remain for the life of the engine.
Tin-nickel electro-deposits have been known for decades, principally as decorative coatings to replace chromium coatings. The deposits are hard, intermediate between nickel and chromium, and have good corrosion resistance. Such coatings generally contain 30 to 35 wt % of nickel and comprise mainly the intermetallic compound, Ni3Sn4.
It is an object of the present invention to provide a lead-free material which may be used as a sliding bearing material for a plain bearing. It is a further object to provide a plain bearing having a lead-free sliding bearing material coating thereon.
According to a first aspect of the present invention there is provided a material for use as a bearing sliding surface, the material comprising tin and having therein from 1 to 5 wt % of a metal selected from the group comprising nickel, cobalt and iron.
In the text that follows references to xe2x80x9cnickelxe2x80x9d also include references to both xe2x80x9ccobaltxe2x80x9d and to xe2x80x9cironxe2x80x9d except where reference is made to particular intermetallic compounds and in the specific examples hereinbelow.
Preferably, the nickel content may be from 1.5 to 3.5 wt %.
More preferably still, the nickel content may be from 2 to 3 wt %.
The lower limit of 1 wt % nickel is chosen because below this level the strengthening effect of the nickel is insufficient to endow adequate fatigue and/or wear resistance. Above 5 wt % nickel, the alloy becomes relatively hard and brittle for use as a soft bearing material, particularly where used as an overlay coating, and also the fatigue strength thereof tends to decline above this level.
The material may be deposited directly on to a strong supporting material backing such as steel but more preferably may be deposited on a layer of another harder (harder than the overlay alloy) bearing material which is itself bonded to a strong backing material such as steel for example. The harder bearing material may be any known bearing material suitable for the intended operating application. However, an aluminium alloy based bearing material is preferred since this combination may provide a completely lead-free bearing.
The material according to the present invention may be deposited by an electroplating route or by a physical vapour deposition route such as cathodic sputtering for example.
The form of the alloy in its as-deposited state is a solid solution of the nickel in the tin matrix. However, this is a metastable form and the stable form is a tin matrix having a dispersion of N3Sn4 intermetallic compound therein. The intermetallic compound does precipitate out at temperatures in excess of 100xc2x0 C. X-ray diffraction work has shown that some Ni3Sn4 is present after 14 hours at 160xc2x0 C. but not after 10 hours at this temperature. However, the size of the precipitate is very small which tends to strengthen the coating providing good fatigue and wear resistance but still endowing the coating with a low hardness of 18 Hk (1 g Knoop indenter) so that the desired properties of conformability and dirt embedability are not impaired. For comparison, the hardness of lead-9 wt % tin is 8Hk and that of lead-10 wt % tin-2 wt % copper is 15 Hk.
When used as a bearing sliding surface directly on a steel backing for example, the thickness may lie in the range from 50 to 75 xcexcm for example. When used as an overlay coating on a substrate of another, harder bearing material, the thickness may lie in the range from about 5 to 50 xcexcm with a range of about 10 to 20 xcexcm being preferred. If used as a thin sacrificial surface coating as described above, the thickness may be in the range from about 1 to 5 xcexcm.
When deposited onto the surface of another bearing alloy there may be an interlayer of pure nickel, copper or copper alloy (eg copper-zinc) for example to act either as a diffusion barrier and/or to enable the bearing material according to the present invention to be more adherent to the surface thereof.
According to a second aspect of the present invention, there is provided a plain bearing having a sliding surface comprising the bearing material of the first aspect of the present invention.